1. Field of the Invention
The present invention concerns an Alxe2x80x94Mgxe2x80x94Si series high strength and high toughness aluminum alloy forgings (aluminum is hereinafter simply referred to as Al) suitable, particularly, to parts for transportation machines such as suspension parts for automobiles.
2. Related Art
As is well-known, Al alloys such as of AA 6XXX series (Alxe2x80x94Mgxe2x80x94Si alloys) excellent in moldability and burn on hardenability have been used as structural materials or suspension parts such as knuckles, lower arms and upper arms for transportation machines of automobiles or vehicles, with an aim of reducing weight. The AA 6XXX series Al alloys are also excellent in other required characteristics such as mechanical properties, for example, formability or corrosion resistance or stress corrosion cracking capability and, in addition, also excellent in view of recycling property capable of re-using scraps as melting materials for AA 6XXX series since they contain less amount of alloying elements such as Mg.
Referring to suspension parts for the automobiles, for example, cast Al alloy materials or Al alloy forgings are used in view of the reduction for the production cost and fabrication into parts of complicate shapes. Among them, Al alloy forgings are used for those parts requiring mechanical properties such as higher strength and higher toughness. The Al alloy forgings are manufactured by soaking a cast alloy material and then applying hot forging such as mechanical forging and tempering such as T6 or aging treatment.
In recent years, it has been demanded for reducing the wall thickness and improving strength for those parts including suspensions for use in automobiles and it has been required also for the Al alloy forgings to improve strength and toughness. However, AA 6XXX series used at present for such application uses, inevitably cause insufficiency in the strength.
In view of the above, it has been proposed to improve strength and toughness of the Al alloy forgings, for example, in Japanese Published Unexamined Patent Application Hei 6-256880 by defining the ingredients of AA 6XXX series (Alxe2x80x94Mgxe2x80x94Si alloys) cast Al alloys for use in forgings used as parts such as suspensions of automobiles, reducing the average grain size as small as to 8 xcexcm or less and reducing the secondary dendrite arm spacing (DAS) as narrow to 40 xcexcm or less.
However, as shown in examples of Japanese Published Unexamined Patent Application Hei 6-256880, the secondary dendrite arm distance (DAS) of the Al alloy forgings obtained in this prior art is about 30 xcexcm at the smallest and the Al alloy forgings has characteristics, as a result of up setting forging test for round bars, for example, a tensile strength ("sgr"B) of about 39.2-39.3 kgf/mm2 (385-394 MPa) and a toughness (Lc) of from 2.2 to 2.3 kgf/mm2 (about 22 J/cm2 as the Charpy impact value) in a case where a forging ratio [(original ingot height doxe2x88x92crack occurring height dt)/do] is 75%.
That is, in the upset forging test for round bars as in the prior art, since each of the portions for a round bar is forged uniformly, mechanical properties are uniform for each of the portions of the round bar. However, as shown in FIG. 2 as an example of Al alloy forgings for use in a suspension part of an automobile, the forging ratio is sometimes lowered depending on the portions of the part even by hot forging such as mechanical forging in an actual Al alloy forgings and mechanical properties are not uniform for each of the portions of the forgings. For example, in a case as shown in FIG. 2, even if the forging ratio is 75% for a portion T1, the forging ratio for the portion T2 is only about 50%. Then, the toughness for the portion with the lower forging ratio is inevitably lowered compared with other portions of higher forging ratio since cast structure remains even after forging.
Then, although the strength and the toughness of the Al alloy forgings obtained by this prior art are improved compared with Al alloys such as of AA 6061 or 6151, average toughness is poor in the Al alloy forgings, particularly, in such an Al alloy forgings in which the toughness for the portion is lowered because of the portion of the lowered forging ratio. That is, in the prior art, the level for the toughness is further lowered at a portion with the forging ratio of 75% or less, further, 50% or less and high yield strength and high toughness values requires for the entire part can not be obtained.
As a result, the forgings can not be applied to parts requiring higher strength and higher toughness as a entire portion and, more specifically, to those parts or members requiring a high strength of 315 N/mm2 or more as "sgr"0.2 and a Charpy impact value of 20 J/cm2 or more as the entire part, and this hinders the development of the Al alloy forgings to the application uses for suspension parts for use in automobiles.
The present invention has been accomplished in view of the foregoing situations and it is an object thereof to provide a high strength and high toughness Al alloy forgings excellent in average mechanical properties as an entire forgings even if a portion with low forging ratio is present, and applicable to those parts or members requiring high strength and high toughness as the entire forgings.
In order to attain the foregoing object, the feature of the Al alloy forgings according to the present invention resides in a high strength and high toughness aluminum alloy forgings containing Mg: 0.6-1.6% (mass % here and hereinafter), Si: 0.6-1.8% and Cu: 0.05-1.0%, Fe: 0.30% or less, one or more of Mn: 0.15-0.6%, Cr: 0.1-0.2% and Zr: 0.05-0.2%, hydrogen: 0.25 cc/100 g Al or less and the balance of Al and inevitable impurities, the Al alloy forgings being prepared by casting a cast Al alloy ingot at a cooling rate of 10xc2x0 C./sec or higher, subjecting the same to a soaking heat treatment at a temperature of 530-600xc2x0 C. and then hot forging into a forgings, in which the volume fraction of total constituents phase particles (Mg2Si, Alxe2x80x94Fexe2x80x94Sixe2x80x94(Mn, Cr, Zr) series intermetallic compounds) is 1.5% or less per unit area.
As a result of a study on the relationship between constituents and the toughness of Al alloy forgings, the present inventors have found that the volume fraction of constituents phase particles has a close concern with the toughness of the Al alloy forgings.
That is, the present inventors have found that, among the constituents of the cast Al alloy materials, Mg2Si and Alxe2x80x94Fexe2x80x94Sixe2x80x94Mn, Alxe2x80x94Fexe2x80x94Sixe2x80x94Cr or Alxe2x80x94Fexe2x80x94Sixe2x80x94Zr series intermetallic compounds constitute starting points for the rupture (starting points for dimples).
More importantly, the present inventors further found that it is not significant that the constituents present in the Al alloy structure are large or of a long chained shape, but that dispersion of them at a spacing with each other contributes to the improvement of the toughness. That is, the constituents can not simply decreased or eliminated since they contribute to the insurance of required strength. However, it has been found that a necessary strength can be insured and a high average toughness can be insured even if the forging ratio is low or even if there is a portion with a low forging ratio, by controlling the form of the constituents that are present inevitably or present by requirement.
For example, no effective contribution can be obtained for the improvement of the toughness by merely controlling the form of the constituents, namely, by merely reducing the average size of the constituents in the cast material as described in Japanese Published Unexampled Patent Application Hei 6-256880. On the contrary to the idea disclosed in Japanese Published Unexamined Patent Application Hei 6-256880, the present inventors have found that the constituents in the cast material, even if their average size is large, can contribute to the improvement of the toughness so long as they are dispersed being spaced apart from each other (present dispersedly). That is, the constituents present densely with a narrow distance between each of them or being continued with each other deteriorate the toughness, particularly, destruction toughness even if the average size of them is small. On the other hand, in the present invention, the amount of the constituents, for example, of Mg2Si and Alxe2x80x94Fexe2x80x94Sixe2x80x94(Mn, Cr, Zr) series intermetallic compounds is controlled or decreased except for the amount ensuring required strength.
Then, the volume fraction of total constituents phase particles (Mg2Si and Alxe2x80x94Fexe2x80x94Sixe2x80x94(Mn, Cr, Zr) series intermetallic compounds) is selected in the present invention as an index well conforming the situation for the control of the amount of constituents and for the situation in which the constituents are dispersed with a spacing between each of them (not a state in which constituents are present densely with a small distance between each of them or present being chained continuously with each other).
The volume fraction of total constituents phase particles is determined by visual observation or by image analyzing observation using a scanning type electron microscope (SEM) at 800 magnification ratio, for the structure of a cast Al alloy or Al alloy forgings in the cross section along the thickness. Referring to the magnification ratio of the scanning type electron microscope, the volume fraction does not change so much when measured at a magnification factor from 400 to 800, but the number of constituents as the object to be measured is quite different in the magnification factor other than the above. Therefore, if the magnification ratio is different, the volume fraction to be measured differs greatly to loss the reproducibility for the definition of the area. Accordingly, in the present invention, the magnification factor of the scanning type electron microscope is determined as 800 as a standard for the definition of the volume fraction. Further, for ensuring reproducibility in the measurement of the volume fraction, it is preferred to observe with the number of field of view (measuring point) for the portion of the object to measure the volume fraction of the constituents being as 5 to 20 fields of view and take an average for the measured volume fraction of the constituents in each of the field of view.
Definition for the constituents in the present invention is to be explained. When the volume fraction of total constituents phase particles (Mg2Si and Alxe2x80x94Fexe2x80x94Sixe2x80x94(Mn, Cr, Zr) series intermetallic compounds) is defined as 1.5% or less, preferably, 1.0% or less per unit area by the visual observation or by image analyzing observation with a scanning type electron microscope (SEM) at 800 magnification ratio, it is possible to obtain higher strength and higher toughness, preferably, a high toughness of 30 J/cm2 or more in an average value with an average value of the yield strength at ("sgr"0.2) being 350 N/mm2 or more required, for example, for suspension parts in use in automobiles.
On the other hand, if the volume fraction of total constituents phase particles exceeds 1.5% per unit area, high average toughness value as the entire part can not be obtained including a case of remarkable lowering of the toughness for a portion of a part in which the forging ratio is lowered even by hot forging (forging ratio of 75% or less).